This invention relates to a resistance welding machine and, more particularly, to a resistance welding machine in which a welding current pattern having any desired waveform can be set, and in which the correlation between the set current pattern and an actual welding current pattern can be checked.
In recent years, inverter-type power supply apparatus for resistance welding machines have spread rapidly in view of their ability to satisfy the demand for welding current controllability, power conservation and smaller size. The scope of utilization of such resistance welding machines extends from large-scale facilities, the foremost of which is an assembly line for automobile parts, to small-scale facilities such as those for bonding electronic parts and integrated chips. Moreover, there are a wide variety of welding materials which can be used, beginning with metal materials that have been surface treated.
Resistance welding involves passing a current at right angles into the contacting portions of metal members to be joined, melting the contacting portions by utilizing the Joule heat produced, and joining the metals by applying pressure. Therefore, in order to achieve high-quality welding of sufficient welding strength without splashing and thermal deformation, the waveform of the current passed through the contacting portions of the metal members is of great importance.
A pattern of the kind shown in FIG. 10 is well known as an example of a current waveform pattern suitable for such resistance welding.
As shown in FIG. 10, the current waveform pattern includes a rising interval T1 corresponding to a preheating stage during which splashing is suppressed and an excellent welding finish obtained, an interval T2 which is the principal current-feed stage for obtaining welding strength, and a decaying interval T3 which is a post-heating stage for eliminating deformation caused by welding heat.
Conventionally, digital switches for time and current corresponding to each interval are provided as means for setting the time and crest value of each interval of the aforementioned current waveform, and the arrangement is such that the time and crest value of each interval is set by manually operating these digital switches to obtain the overall current waveform shown in FIG. 10.
In judging the success or failure of a welding operation when carrying out welding in accordance with the welding current command pattern set as described above, the crest value at the time of principal current feed shown in FIG. 10 is measured and the acceptability of the welding operation is judged depending upon whether the measured value falls within a preset range of set values.
With the resistance welding machine of the kind set forth above, the welding current is set by the digital switches, which means that only one welding current pattern can be set by one set of digital switches. Accordingly, when it is attempted to set a plurality of welding current patterns, it is necessary to provide a plurality of digital switch sets. This is attended by an increase in the number of digital switches, an increase in the size of the setting apparatus, a more complicated and laborious operation for setting the welding current value and higher cost. Thus, the conventional arrangement is far from practical.
Accordingly, the state of the art is such that a uniform welding current waveform is set by one set of digital switches in the conventional digital switch configuration.
When judging the acceptability of the welding results in conventional resistance welding, the method adopted is to detect the crest value at principal current feed and perform a comparison to determine whether the detected value lies within a preset range of reference values. If the detected value lies within the range of reference values, therefore, any abnormalities that might develop, such as splashing on the surface of the welded object after welding or the occurrence of thermal deformation, cannot be detected at the welding stage.